1. Field of the Invention
The present invention relates to a vacuum pump, and more particularly to a vacuum pump that can be used in a pressure range from medium vacuum to high vacuum and ultra-high vacuum, in an industrial vacuum system used in semiconductor manufacturing, high-energy physics and the like.
2. Description of the Related Art
Conventional vacuum pumps of this type have a structure wherein a turbo-molecular pump section and a cylindrical thread groove pump section are sequentially disposed inside a chassis that has an intake port and an exhaust port.
The rotor or stator at the cylindrical thread groove pump section is made of an aluminum alloy. Thus, the raise of vacuum pump revolution speed is limited by the strength of the rotor at the cylindrical thread groove pump section.
Such being the case, a cylindrical rotor that results from shaping, to a cylindrical shape, a fiber-reinforced plastic material (fiber-reinforced plastic, ordinarily referred to as “FRP material”), may be used as the rotor in the thread groove pump section of the vacuum pump. Structures for increasing the strength of such a cylindrical rotor are also known. When in rotation, the cylindrical rotor is acted upon, in the circumferential direction, by a load that results from differences in centrifugal force and between coefficients of thermal expansion. In the case of FRP, therefore, a layer in which the fibers are aligned along the circumferential direction is ordinarily formed on the outermost side. As the fiber-reinforced plastic material there can be used, for instance, aramid fibers, boron fibers, carbon fibers, glass fibers, polyethylene fibers and the like.
In a case where the fiber-reinforced plastic material (hereafter, FRP material) is shaped in the form of a cylinder to yield a cylindrical rotor, the surface after shaping of the FRP material to a cylindrical shape is significantly distorted, and hence finish processing is required after shaping. However, the meandering fibers in the vicinity of the surface layer of the cylindrical rotor are shredded during this finish processing. When acted upon by a high load, therefore, the fibers in the FRP material may partially peel off, become frayed and/or distorted, and be damaged as a result.
Conventional measures against the above occurrences have been proposed in, for instance, Japanese Patent Publication No. 3098139 and Japanese Patent Application Publication No. 2004-278512.
In a vacuum pump of Japanese Patent Publication No. 3098139, specifically, a rotor of a turbo-molecular pump section and a cylindrical rotor of a thread groove pump section are joined to each other by way of a support plate of FRP material, in order to mitigate the difference in the extent of deformation caused by centrifugal force and by differences in thermal expansion between the turbo-molecular pump section and the thread groove pump section.
In the vacuum pump disclosed in Japanese Patent Application Publication No. 2004-278512, the winding angle of fibers of an FRP material, as well as shapes and shaping conditions, such as resin content, are so designed as to mitigate the difference in the extent of deformation caused by centrifugal force and differences in thermal expansion between the turbo-molecular pump section and the thread groove pump section.
The structure disclosed in Japanese Patent Publication No. 3098139, wherein the rotor in the turbo-molecular pump section and the cylindrical rotor in the thread groove pump section are joined to each other by way of a support plate of a FRP material, as a measure against the occurrence of fiber fraying and distortion and resulting damage of fibers, in a cylindrical rotor that is obtained by shaping a conventional FRP material to a cylindrical shape, as described above, is problematic structure on account of the increased number of parts and greater assembly man-hours that such a structure involves. In some instances, moreover, assembly is difficult to achieve with good precision, and the clearance with respect to a fixed section must be widened in order to prevent contact with the fixed section. This entails lower evacuation performance, which is likewise problematic.
In a structure as disclosed in Japanese Patent Application Publication No. 2004-278512, i.e., a structure in which the winding angle of fibers of an FRP material, and shaping shapes and conditions, such as resin content, are variously designed, the shape of the FRP material is complex, which is problematic in terms of poorer productivity and higher costs.